Communication system, communication control method, mobile terminal, and relay device

ABSTRACT

A communication system is configured that includes a mobile terminal, one or more relay devices, and a base station for communicating with the mobile terminal via any of the one or more relay devices, where the mobile terminal includes a reception unit for receiving a reference signal transmitted from the one or more relay devices, a determination unit for determining a relay device to be used for communication with the base station based on the reference signal received by the reception unit from the one or more relay devices, and a transmission unit for transmitting to the base station information indicating the relay device determined by the determination unit.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/319,098, filed Nov. 7, 2011, the entire contents of which areincorporated herein by reference. U.S. application Ser. No. 13/319,098is a National Stage of PCT/JP10/59852, filed Jun. 10, 2010, and claimsthe benefit of priority under 35 U.S.C. §119 of Japanese Application No.2009-172492, filed Jul. 23, 2009.

TECHNICAL FIELD

The present invention relates to a communication system, a communicationcontrol method, a mobile terminal, and a relay device.

BACKGROUND ART

In 3GPP (Third Generation Partnership Project), a technology that uses arelay device (relay station) to realize an increase in the throughput atthe cell edge is being actively considered.

This relay device receives, in a downlink, a signal transmitted from abase station, amplifies the same, and then transmits the amplifiedsignal to a mobile terminal. By performing such relaying, the relaydevice can increase the signal-to-noise ratio than when directlytransmitting a signal from the base station to the mobile terminal.Similarly, in an uplink, the relay device can maintain highsignal-to-noise ratio by relaying a signal transmitted from the mobileterminal to the base station.

Additionally, as a relay scheme of the relay device, an Amp-Forwardtype, a Decode-Forward type, and the like can be cited. The Amp-Forwardtype is a scheme of amplifying and transmitting a received signal whilekeeping it as an analogue signal. According to this Amp-Forward type,although the signal-to-noise ratio is not improved, there is anadvantage that the communication protocol does not have to be refined.Additionally, the relay device has a feedback path between atransmission antenna and a reception antenna, and is designed such thatthe feedback path does not oscillate.

The Decode-Forward type is a scheme of converting a received signal to adigital signal by AD conversion, performing decoding such as errorcorrection on the digital signal, encoding again the decoded digitalsignal, converting the digital signal to an analogue signal by DAconversion, amplifying the analogue signal, and transmitting the same.According to the Decode-Forward type, the signal-to-noise ratio can beimproved by a coding gain. Also, by storing a digital signal obtained byreception in a memory and transmitting the digital signal in the nexttime slot, the relay device can avoid oscillation of a feedback pathbetween a transmission antenna and a reception antenna. Additionally,the relay device is also capable of avoiding the oscillation by changingthe frequency instead of the time slot.

Furthermore, since it is assumed that there are a plurality of relaydevices present within the cell provided by a base station, it iseffective from the standpoint of power consumption reduction toimplement a power save mode (sleep mode) in the relay devices.Additionally, the power save mode in LTE (Long Term Evolution) isdescribed in Non-Patent Literature 1, for example.

CITATION LIST Non-Patent Literature

-   Non-Patent Literature 1: Erik Dahlman, Stefan ParkVall, et al, “3G    Evolution: HSPA and LTE for Mobile Broadband”, 2007, p. 314

SUMMARY OF INVENTION Technical Problem

However, if relay devices operating in the power save mode do nottransmit radio signals, a mobile terminal is not able to receive radiosignals from the relay devices operating in the power save mode, andthus it is difficult to appropriately determine a relay device to beused for the communication between a base station and the mobileterminal.

Accordingly, the present invention is made in view of the above problem,and the object of the present invention is to provide a communicationsystem, a communication control method, a mobile terminal, and a relaydevice which are novel and improved, and which are capable ofdetermining a relay device to be used for the communication between thebase station and the mobile terminal from one or more relay deviceshaving a power save mode.

Solution to Problem

According to an aspect of the present invention, in order to achieve theabove-mentioned object, there is provided a communication systemincluding a mobile terminal, one or more relay devices, and a basestation for communicating with the mobile terminal via any of the one ormore relay devices, where the mobile terminal includes a reception unitfor receiving a reference signal transmitted from the one or more relaydevices, a determination unit for determining a relay device to be usedfor communication with the base station based on the reference signalreceived by the reception unit from the one or more relay devices, and atransmission unit for transmitting to the base station informationindicating the relay device determined by the determination unit.

Each of the one or more relay devices may perform transmission of thereference signal in an active mode where relaying of communicationbetween the base station and the mobile terminal is enabled and in apower save mode where intermittent reception is performed.

The base station may include a mode control unit for instructing, wheninformation indicating the relay device is received from the mobileterminal, the relay device to shift to the active mode, in a case therelay device is operating in the power save mode.

Each of the one or more relay devices does not have to transmit thereference signal when operating in the power save mode and may transmitthe reference signal when operating in the active mode, and the basestation may include a terminal location acquisition unit for acquiringlocation information of the mobile terminal, a selection unit forselecting a relay device among the one or more relay devices based onthe location information of the mobile terminal acquired by the terminallocation acquisition unit, and a mode control unit for issuing aninstruction to shift to the active mode, in a case the relay deviceselected by the selection unit is operating in the power save mode.

The terminal location acquisition unit may acquire the locationinformation of the mobile terminal based on an arrival direction and areception intensity of a signal transmitted from the mobile terminal.

The mobile terminal may transmit to the base station reception intensityinformation of each signal transmitted from a plurality of basestations, and the terminal location acquisition unit may acquire thelocation information of the mobile terminal based on the receptionintensity information of each of the plurality of base stations receivedfrom the mobile terminal.

A plurality of base stations may transmit to the base station receptionintensity information of a signal transmitted by the mobile terminal,and the terminal location acquisition unit may acquire the locationinformation of the mobile terminal based on reception intensities of thesignal transmitted by the mobile terminal at the plurality of basestations.

The mobile terminal may transmit to the base station reception intensityinformation of each signal transmitted from the base station and the oneor more relay devices, and the terminal location acquisition unit mayacquire the location information of the mobile terminal based on thereception intensity information received from the mobile terminal.

The mobile terminal may transmit to the base station the locationinformation of the mobile terminal estimated by GPS, and the terminallocation acquisition unit may acquire the location information of themobile terminal transmitted from the mobile terminal.

According to another aspect of the present invention, in order toachieve the above-mentioned object, there is provided a communicationcontrol method including the steps of receiving, by a mobile terminal, areference signal transmitted from one or more relay devices, determininga relay device to be used for communication with a base station based onthe reference signal received from the one or more relay devices, andtransmitting to the base station information indicating the relay devicewhich has been determined.

According to another aspect of the present invention, in order toachieve the above-mentioned object, there is provided a mobile terminalincluding a reception unit for receiving a reference signal transmittedfrom one or more relay devices, a determination unit for determining arelay device to be used for communication with a base station based onthe reference signal received by the reception unit from the one or morerelay devices, and a transmission unit for transmitting to the basestation information indicating the relay device determined by thedetermination unit.

According to another aspect of the present invention, in order toachieve the above-mentioned object, there is provided a relay device.The relay device relays communication between a mobile terminalincluding a reception unit for receiving a reference signal transmittedfrom one or more relay devices, a determination unit for determining arelay device to be used for communication with a base station based onthe reference signal received by the reception unit from the one or morerelay devices, and a transmission unit for transmitting to the basestation information indicating the relay device determined by thedetermination unit, and the base station.

Advantageous Effects of Invention

As described above, according to the present invention, a relay deviceto be used for the communication between a base station and a mobileterminal can be determined from one or more relay devices having a powersave mode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram showing a configuration of acommunication system according to an embodiment of the presentinvention.

FIG. 2 is an explanatory diagram showing each link in the communicationsystem according to the embodiment of the present invention.

FIG. 3 is an explanatory diagram showing an example configuration of aradio frame used in the communication system according to theembodiment.

FIG. 4 is an explanatory diagram showing operation modes of a mobileterminal according to LTE.

FIG. 5 is a functional block diagram showing a configuration of a relaydevice according to a first embodiment.

FIG. 6 is an explanatory diagram showing an operation of a relay devicein sleep mode.

FIG. 7 is a functional block diagram showing a configuration of a mobileterminal according to the first embodiment.

FIG. 8 is an explanatory diagram showing reception states, at a mobileterminal 20, of reference signals transmitted from a plurality of relaydevices.

FIG. 9 is a functional block diagram showing a configuration of a basestation according to the first embodiment.

FIG. 10 is a sequence chart showing an operation of a communicationsystem according to the first embodiment.

FIG. 11 is a functional block diagram showing a configuration of a relaydevice according to a second embodiment.

FIG. 12 is a functional block diagram showing a configuration of amobile terminal according to the second embodiment.

FIG. 13 is a functional block diagram showing a configuration of a basestation according to the second embodiment.

FIG. 14 is a sequence chart showing an operation of a communicationsystem according to the second embodiment.

FIG. 15 is an explanatory diagram showing a modified example 1 of anacquisition method of location information of a mobile terminal.

FIG. 16 is an explanatory diagram showing a modified example 2 of anacquisition method of location information of a mobile terminal.

FIG. 17 is an explanatory diagram showing a modified example 3 of anacquisition method of location information of a mobile terminal.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings. Note that,in this specification and the drawings, elements that have substantiallythe same function and structure are denoted with the same referencesigns, and repeated explanation is omitted.

Also, in this specification and the drawings, a plurality of structuralelements having substantially the same functional configuration may bedistinguished from each other by each having a different letter added tothe same reference numeral. For example, a plurality of elements havingsubstantially the same functional configuration are distinguished fromeach other as necessary as mobile terminals 20A, 20B, and 20C. However,if it is not particularly necessary to distinguish each of a pluralityof structural elements having substantially the same functionalconfiguration, only the same reference numeral is assigned. For example,if it is not particularly necessary to distinguish between the mobileterminals 20A, 20B, and 20C, they are simply referred to as the mobileterminal 20.

Furthermore, the “Description of Embodiments” will be describedaccording to the following item order.

1. Overview of Communication System

2. First Embodiment

-   -   2-1. Configuration of Relay Device    -   2-2. Configuration of Mobile Terminal    -   2-3. Configuration of Base Station    -   2-4. Operation of Communication System

3. Second Embodiment

-   -   3-1. Configuration of Relay Device    -   3-2. Configuration of Mobile Terminal    -   3-3. Configuration of Base Station    -   3-4. Operation of Communication System    -   3-5. Modified Example 1    -   3-6. Modified Example 2    -   3-7. Modified Example 3    -   3-8. Modified Example 4

4. Summary

1. Overview of Communication System

First, a communication system 1 according to an embodiment of thepresent invention will be briefly described with reference to FIGS. 1 to4.

FIG. 1 is an explanatory diagram showing the configuration of thecommunication system 1 according to the embodiment of the presentinvention. As shown in FIG. 1, the communication system 1 according tothe embodiment of the present invention includes a plurality of basestations 10A, 10B, and 10C, a backbone network (backhaul) 12, aplurality of mobile terminals 20A, 20B, and 20C, and a plurality ofrelay devices 30A, 30B, 30C, and 30D.

The plurality of base stations 10A, 10B, and 10C manage communicationwith the mobile terminals 20 that are present in their radio wavecoverages. For example, the base station 10A manages a communicationschedule for the mobile terminal 20C present in the radio wave coverageof the base station 10A, and communicates with the mobile terminal 20Caccording to this communication schedule.

Furthermore, the plurality of base stations 10A, 10B, and 10C are alsocapable of communicating with the mobile terminals 20 present in theirradio wave coverages via the relay device 30. In this case, theplurality of base station 10A, 10B, and 10C manage the communicationschedule for the relay device 30 and the communication schedule forbetween the relay device 30 and the mobile terminal 20. For example, thebase station 10A manages the communication schedule for the relay device30A present in the radio wave coverage of the base station 10A and thecommunication schedule for between the relay device 30A and the mobileterminals 20A and 20B.

Additionally, in the present specification, an explanation will be givenon the management of the communication schedule, placing emphasis oncentralized control performed by the base station 10, but the presentinvention is not limited to such an example. For example, thecommunication schedule may be managed by the relay device 30(distributed scheduling).

Furthermore, the plurality of base stations 10A, 10B, and 10C areconnected via the backbone network 12. The plurality of base stations10A, 10B, and 10C are capable of exchanging various information forcommunication via this backbone network 12.

The relay device 30 relays the communication between the base station 10and the mobile terminal 20. Specifically, in the downlink, the relaydevice 30 receives a signal transmitted from the base station 10, andtransmits the amplified signal to the mobile terminal 20. By performingsuch relaying, the relay device 30 can increase the signal-to-noiseratio than when directly transmitting the signal from the base station10 to the mobile terminal 20 near the cell edge.

Similarly, also in the uplink, the relay device 30 relays a signaltransmitted from the mobile terminal 20 to the base station 10, andthereby maintains a high signal-to-noise ratio. Additionally, an exampleis shown in FIG. 1 where only the relay device 30A is present in thecell provided by the base station 10A, but a plurality of relay devices30 may be present in the cell provided by the base station 10A. Linknames will now be organized with reference to FIG. 2.

FIG. 2 is an explanatory diagram showing each link in the communicationsystem 1 according to the embodiment of the present invention. As shownin FIG. 2, a direct communication path between the base station 10 andthe mobile terminal 20 is referred to as a direct link. Also, thecommunication path between the base station 10 and the relay device 30is referred to as a relay link, and the communication path between therelay device 30 and the mobile terminal 20 is referred to as an accesslink.

As described above, the mobile terminal 20 communicates with the basestation 10 directly or via the relay device 30. Additionally, as thedata to be transmitted/received by the mobile terminal 20, audio data,music data such as music, a lecture, a radio program, or the like, stillimage data such as a photograph, a document, a painting, a diagram, orthe like, video data such as a movie, a television program, a videoprogram, a game image, or the like, may be cited.

Now, the configuration of a radio frame used in the communication system1 according to the present embodiment will be described with referenceto FIG. 3.

FIG. 3 is an explanatory diagram showing an example configuration of aradio frame used in the communication system 1 according to the presentembodiment. As shown in FIG. 3, the length of each radio frame is 10 ms.Also, each radio frame is formed from ten subframes #0 to #9 whoselengths are 1 ms.

Also, each subframe is formed from two 0.5 ms slots, and each 0.5 msslot is formed from seven OFDM (orthogonal frequency divisionmultiplexing) symbols.

Also, the fifth and sixth OFDM symbols of the first 0.5 ms slotsincluded in the subframes #0 and #5 are used for transmission ofreference signals for synchronization. The mobile terminal 20 performs acell search and a synchronization process based on this reference signaltransmitted from the base station 10 or the relay station 30.

Additionally, the base station 10 allots time on a per-0.5 ms slot basisfor communication with the mobile terminal 20. Furthermore, to separatethe uplink and the downlink, FDD (Frequency Division Duplex) and TDD(Time Division Duplex) are used.

Next, operation modes of the mobile terminal 20 and the relay device 30will be described.

(Operation Mode of Mobile Terminal)

FIG. 4 is an explanatory diagram showing operation modes of the mobileterminal 20 according to LTE. As shown in FIG. 4, the operation modesinclude LTE_DETACHED, LTE_ACTIVE, and LTE_IDLE.

After activation, the mobile terminal 20 goes into an operation modecalled LTE_DETACHED. In LTE_DETACHED, an IP address is not assigned, andthe belonging cell is unknown.

Then, the mobile terminal 20 shifts to LTE_ACTIVE (active mode). In theactive mode, cell synchronization and setting of an IP address for themobile terminal are performed. Also, both a state where the cellsynchronization is achieved and a state where it is not achieved areincluded in the active mode. The mobile terminal 20 is capable ofcommunication regardless of uplink or downlink as long as it is in astate where the cell synchronization is achieved.

Furthermore, to reduce power consumption, the mobile terminal 20 shiftsfrom the active mode to LTE_IDLE (sleep mode). In the sleep mode, themobile terminal 20 performs DRX (Discontinuous Reception). That is, themobile terminal 20 intermittently receives, according to a predeterminedcycle, signals transmitted from the base station 10. Therefore,according to the sleep mode, power consumption can be reduced.

When the mobile terminal 20 operates in this sleep mode, although the IPaddress is set, the network side including the base station 10 is notallowed to perceive the cell to which the mobile terminal 20 belongs.The network side perceives the location of the mobile terminal 20 at thegranularity of a plurality of cells called a tracking area. Accordingly,the network side transmits to the cells in the tracking area, accordingto a DRX cycle, L1/L2 signaling in which paging information for pagingthe mobile terminal 20 is described.

On the other hand, the mobile terminal 20 receives, according to the DRXcycle, L1/L2 signaling in which downlink control information such as thepaging information and the schedule information are described. Then, themobile terminal 20 decides whether the mobile terminal 20 is beingpaged, that is, whether communication addressed to the mobile terminal20 exists, based on the paging information or the schedule information,and shifts to the active mode if necessary.

Additionally, LTE-Advanced discussed in 3GPP is considered based on LTE.LTE is a communication scheme that is based on an OFDM demodulationscheme. Since OFDM uses subcarriers, each mobile terminal 20 can performcommunication while avoiding interference by using a differentsubcarrier or a different time slot. More particularly, according toLTE, one resource block is defined by twelve subcarriers and seven OFDMsymbols. Communication resources are allocated to each mobile terminal20 on a per-resource-block basis.

(Operation Mode of Relay Device)

According to the present embodiment, since there are a plurality ofrelay devices 30 present in the cell provided by the base station 10,the sleep mode is implemented also in the relay devices 30 from thestandpoint of power consumption reduction. In the case of operating inthe sleep mode, the relay device 30 receives, according to the DRXcycle, L1/L2 signaling transmitted from the base station 10.

Furthermore, in the case of operating in the active mode, the relaydevice 30 decides presence or absence of communication addressed to therelay device 30 by referring to the schedule information included in theL1/L2 signaling transmitted at 1 ms intervals. Then, in the case thereis no communication addressed to the relay device 30, the relay device30 can be shifted to the sleep mode over a period when there is nocommunication addressed to the relay device 30.

Additionally, the distributed scheduling is also based on the schedulingby the base station 10. Accordingly, in the case neither the uplink northe downlink is specified for the relay device 30 by the scheduling bythe base station 10, neither the access link nor the relay link is used.Therefore, in the case neither the uplink nor the downlink is specifiedfor the relay device 30, the direct link may exist but communication viathe relay device 30 at least will not be performed, and the relay device30 can shift to the sleep mode.

Background of Present Embodiment

The mobile terminal 20 determines its belonging base station 10 based oncell synchronization or reception intensity of signals transmitted fromthe base stations 10. Specifically, the mobile terminal 20 performssynchronization processing and measurement of reception intensity byusing the reference signals transmitted in the fifth and sixth OFDMsymbols in the first 0.5 ms slots included in subframes #0 and #5 shownin FIG. 3.

In the case of operating in the same way as LTE, a method is conceivableof the mobile terminal 20 receiving the reference signals transmittedfrom respective relay devices 30 and selecting the relay device 30 to beused based on the reception intensity of the reference signals.Additionally, the relay device 30 may transmit the reference signal inthe slot designated by LTE or in another slot.

However, if the relay devices 30 operating in the sleep mode do nottransmit reference signals, the mobile terminal 20 is not able toreceive the reference signals from the relay devices 30 operating in thesleep mode, and thus it is difficult to appropriately select a relaydevice 30.

Furthermore, the reference as to whether a relay device 30 is to be usedor not may be as follows.

If, when comparing the reception intensity of a signal transmitted bythe mobile terminal 20 at the relay device 30 and at the base station10, the reception intensity at the former is higher, the significance ofperforming relaying is great.

If the reception intensity at the base station 10 is higher, there isnot much significance in performing relaying.

Also, if the difference between both reception intensities is small,there is not much significance in performing relaying.

Furthermore, even if the difference between both reception intensitiesis great, there is not much significance in performing relaying in thecase the reception intensity at the base station 10 is sufficientlyhigh. Additionally, the case the reception intensity is sufficientlyhigh is a case where sufficient SNR can be obtained compared to therequired SNR of the demodulation scheme that is used.

That is, if the relay device 30 is in the active mode, the base station10 can decide whether use of the relay device 30 is effective or not bycomparing the reception intensity at the relay device 30 and thereception intensity at the base station 10.

However, if the relay device 30 is in the sleep mode and the relaydevice 30 is not able to receive the signal transmitted by the mobileterminal 20, the base station 10 is not able to receive the receptionintensity at the relay device 30, and thus it is difficult toappropriately decide whether use of the relay device 30 is effective ornot.

Accordingly, the first embodiment and the second embodiment of thepresent invention have been achieved in view of the above-describedbackground. According to the first embodiment and the second embodiment,a relay device 30 to be used for communication between the base station10 and the mobile terminal 20 can be determined from one or more relaydevices 30 having the sleep mode. In the following, the details of thefirst embodiment and the second embodiment will be described withreference to FIGS. 5 to 17.

2. First Embodiment 2-1. Configuration of Relay Device

FIG. 5 is a functional block diagram showing the configuration of therelay device 30 according to the first embodiment. As shown in FIG. 5,the relay device 30 includes a plurality of antennas 320 a to 320 n, ananalogue processing unit 324, an AD/DA converter 328, and a digitalprocessing unit 330, and performs L2 relay.

Each of the plurality of antennas 320 a to 320 n receives a radio signalfrom the base station 10 or the mobile terminal 20 and acquires anelectrical high-frequency signal, and supplies the high-frequency signalto the analogue processing unit 324. Also, each of the plurality ofantennas 320 a to 320 n transmits a radio signal to the base station 10or the mobile terminal 20 based on the high-frequency signal suppliedfrom the analogue processing unit 324. Since the relay device 30 isprovided with the plurality of antennas 320 a to 320 n as described, itis capable of performing MIMO (Multiple Input Multiple Output)communication or diversity communication.

The analogue processing unit 324 converts the high-frequency signalssupplied from the plurality of antennas 320 a to 320 n into basebandsignals by performing analogue processing such as amplification,filtering, down-conversion, or the like. Also, the analogue processingunit 324 converts a baseband signal supplied from the AD/DA converter328 into a high-frequency signal.

The AD/DA converter 328 converts the analogue baseband signal suppliedfrom the analogue processing unit 324 into a digital format, andsupplies the same to the digital processing unit 330. Also, the AD/DAconverter 328 converts a digital baseband signal supplied from thedigital processing unit 330 into an analogue format, and supplies thesame to the analogue processing unit 324.

The digital processing unit 330 includes a synchronization unit 332, adecoder 334, a buffer 338, an encoder 340, a control unit 342, a DRXcycle holding unit 344, and a DTX cycle holding unit 346. Among these,the synchronization unit 332, the decoder 334, the encoder 340, and thelike function, together with the plurality of antennas 320 a to 320 n,the analogue processing unit 324, and the AD/DA converter 328, as atransmission unit and a reception unit for communicating with the basestation 10 and the mobile terminal 20.

The synchronization unit 332 is supplied, from the AD/DA converter 328,with a reference signal transmitted from the base station 10, andperforms a synchronization process of a radio frame based on thereference signal. Specifically, the synchronization unit 332 performssynchronization of the radio frame by computing the correlation betweenthe reference signal and a known sequence pattern and detecting the peakposition of the correlation.

The decoder 334 decodes a baseband signal supplied from the AD/DAconverter 328 and obtains relay data for the base station 10 or themobile terminal 20. Additionally, the decoding may include a MIMOreception process, an OFDM demodulation process, an error correctionprocess, and the like, for example.

The buffer 338 temporarily holds the relay data, obtained by the decoder334, for the base station 10 or the mobile terminal 20. Then, the relaydata for the mobile terminal 20 is read out, by the control of thecontrol unit 342, from the buffer 338 to the encoder 340 in thetransmission time of the access downlink to the mobile terminal 20.Likewise, the relay data for the base station 10 is read out, by thecontrol of the control unit 342, from the buffer 338 to the encoder 340in the transmission time of the relay uplink to the base station 10.

The encoder 340 encodes the data supplied from the buffer 338, andsupplies the same to the AD/DA converter 328. Additionally, the encodingmay include a MIMO transmission process and an OFDM demodulationprocess, for example.

The control unit 342 controls transmission processing, receptionprocessing, shifting of the operation mode, or the like of the relaydevice 30. Specifically, in the case the relay device 30 operates in thesleep mode, the control unit 342 controls the relay device 30 such thatL1/L2 signaling is received according to the DRX cycle held in the DRXcycle holding unit 344. Then, the control unit 342 refers to the L1/L2signaling decoded by the decoder 334, and in the case there iscommunication addressed to the relay device 30, shifts the operationmode to the active mode. The control unit 342 shifts the operation modeto the active mode also in the case an instruction to shift to theactive mode is received from the base station 10.

Also, in the case the relay device 30 operates in the sleep mode, thecontrol unit 342 controls the relay device 30 such that the referencesignal is transmitted according to the DTX (Discontinuous Transmission)cycle held in the DTX cycle holding unit 346.

FIG. 6 is an explanatory diagram showing an operation of the relaydevice 30 in the sleep mode. As shown in FIG. 6, in the sleep mode, therelay device 30 receives the L1/L2 signaling according to the DRX cycle(L1/L2 Rx) and transmits the reference signal (Ref Tx) according to theDTX cycle.

More particularly, the relay device 30 may perform transmission of thereference signal in the fifth and sixth OFDM symbols in the first 0.5 msslots included in subframes #0 and #5. Additionally, the relay device 30may also transmit the reference signal only in one radio frame among tenradio frames, for example, instead of transmitting the reference signalin all the radio frames.

Also, each of the relay devices 30 transmits the reference signal usinga different spreading code. Accordingly, the mobile terminal 20 isallowed to receive the reference signal from each relay device 30 bycode division and to identify the relay device 30 which is thetransmission source based on the spreading code that is being used.

2-2. Configuration of Mobile Terminal

Next, the configuration of the mobile terminal 20 will be described withreference to FIGS. 7 and 8.

FIG. 7 is a functional block diagram showing the configuration of themobile terminal 20 according to the first embodiment. As shown in FIG.7, the mobile terminal 20 includes a plurality of antennas 220 a to 220n, an analogue processing unit 224, an AD/DA converter 228, and adigital processing unit 230.

Each of the plurality of antennas 220 a to 220 n receives a radio signalfrom the base station 10 or the relay device 30 and acquires anelectrical high-frequency signal, and supplies the high-frequency signalto the analogue processing unit 224. Also, each of the plurality ofantennas 220 a to 220 n transmits a radio signal to the base station 10or the relay device 30 based on the high-frequency signal supplied fromthe analogue processing unit 224. Since the mobile terminal 20 isprovided with the plurality of antennas 220 a to 220 n as described, itis capable of performing MIMO communication or diversity communication.

The analogue processing unit 224 converts the high-frequency signalssupplied from the plurality of antennas 220 a to 220 n into basebandsignals by performing analogue processing such as amplification,filtering, down-conversion, or the like. Also, the analogue processingunit 224 converts a baseband signal supplied from the AD/DA converter228 into a high-frequency signal.

The AD/DA converter 228 converts the analogue baseband signal suppliedfrom the analogue processing unit 224 into a digital format, andsupplies the same to the digital processing unit 230. Also, the AD/DAconverter 228 converts a digital baseband signal supplied from thedigital processing unit 230 into an analogue format, and supplies thesame to the analogue processing unit 224.

The digital processing unit 230 includes a synchronization unit 232, adecoder 234, a relay device determination unit 236, a transmission datageneration unit 238, an encoder 240, a control unit 242, a DRX cycleholding unit 244, and a DTX cycle holding unit 246. Among these, thesynchronization unit 232, the decoder 234, the encoder 240, and the likefunction, together with the plurality of antennas 220 a to 220 n, theanalogue processing unit 224, and the AD/DA converter 228, as atransmission unit and a reception unit for communicating with the basestation 10 and the relay device 30.

The synchronization unit 232 is supplied, from the AD/DA converter 228,with a reference signal transmitted from the base station 10 or therelay device 30, and performs a synchronization process of a radio framebased on the reference signal. Specifically, the synchronization unit232 performs synchronization of the radio frame by computing thecorrelation between the reference signal and a known sequence patternand detecting the peak position of the correlation.

The decoder 234 decodes a baseband signal supplied from the AD/DAconverter 228 and obtains received data. Additionally, the decoding mayinclude a MIMO reception process and an OFDM demodulation process, forexample.

The relay device determination unit 236 determines a relay device 30 tobe used for communication with the base station 10 from a plurality ofrelay devices 30 based on the magnitudes of the correlations of thereference signals obtained by the synchronization unit 232.Specifically, the relay device determination unit 236 may determine therelay device 30 which is the transmission source of the reference signalwith the highest correlation as the relay device 30 to be used for thecommunication with the base station 10.

The transmission data generation unit 238 is supplied with informationindicating the relay device 30 determined by the relay devicedetermination unit 236, generates transmission data including thisinformation, and supplies the same to the encoder 240.

The encoder 240 encodes the transmission data supplied from thetransmission data generation unit 238, and supplies the same to theAD/DA converter 228. Additionally, the encoding may include a MIMOtransmission process and an OFDM demodulation process, for example.

The control unit 242 controls transmission processing, receptionprocessing and shifting of the operation mode of the mobile terminal 20.For example, in the case the mobile terminal 20 operates in the sleepmode, the control unit 242 controls the mobile terminal 20 such thatL1/L2 signaling is received according to the DRX cycle held in the DRXcycle holding unit 244. Then, the control unit 242 refers to the L1/L2signaling decoded by the decoder 234, and in the case there iscommunication addressed to the mobile terminal 20, shifts the operationmode to the active mode.

Furthermore, in the case of operating in the sleep mode, the controlunit 242 performs control such that the mobile terminal 20 performsreception processing according to the DTX cycle held in the DTX cycleholding unit 246 to thereby receive the reference signal transmittedfrom the relay device 30 (including the relay device 30 operating in thesleep mode).

FIG. 8 is an explanatory diagram showing reception states, at the mobileterminal 20, of reference signals transmitted from a plurality of relaydevices 30. As shown in FIG. 8, the mobile terminal 20 receives thereference signals from the relay devices 30A and 30B operating in theactive mode at 5-subframe (5 ms) intervals. On the other hand, themobile terminal 20 receives the reference signals from the relay device30C operating in the sleep mode at 15-subframe (15 ms) intervals, forexample, according to the DTX cycle held in the DTX cycle holding unit246.

2-3. Configuration of Base Station

Next, the configuration of the base station 10 according to the firstembodiment will be described with reference to FIG. 9.

FIG. 9 is a functional block diagram showing the configuration of thebase station 10 according to the first embodiment. As shown in FIG. 9,the base station 10 includes a plurality of antennas 120 a to 120 n, ananalogue processing unit 124, an AD/DA converter 128, and a digitalprocessing unit 130.

Each of the plurality of antennas 120 a to 120 n receives a radio signalfrom the relay device 30 or the mobile terminal 20 and acquires anelectrical high-frequency signal, and supplies the high-frequency signalto the analogue processing unit 124. Also, each of the plurality ofantennas 120 a to 120 n transmits a radio signal to the relay device 30or the mobile terminal 20 based on the high-frequency signal suppliedfrom the analogue processing unit 124. Since the base station 10 isprovided with the plurality of antennas 120 a to 120 n as described, itis capable of performing MIMO communication or diversity communication.

The analogue processing unit 124 converts the high-frequency signalssupplied from the plurality of antennas 120 a to 120 n into basebandsignals by performing analogue processing such as amplification,filtering, down-conversion, or the like. Also, the analogue processingunit 124 converts a baseband signal supplied from the AD/DA converter128 into a high-frequency signal.

The AD/DA converter 128 converts the analogue baseband signal suppliedfrom the analogue processing unit 124 into a digital format, andsupplies the same to the digital processing unit 130. Also, the AD/DAconverter 128 converts a digital baseband signal supplied from thedigital processing unit 130 into an analogue format, and supplies thesame to the analogue processing unit 124.

The digital processing unit 130 includes a decoder 134, an encoder 140,and a control unit 142. Among these, the decoder 134, the encoder 140,and the like function, together with the plurality of antennas 120 a to120 n, the analogue processing unit 124, and the AD/DA converter 128, asa transmission unit and a reception unit for communicating with therelay device 30 and the mobile terminal 20.

The decoder 134 decodes a baseband signal supplied from the AD/DAconverter 128 and obtains received data. Additionally, the decoding mayinclude a MIMO reception process, an OFDM demodulation process, an errorcorrection process, and the like, for example.

The encoder 140 encodes transmission data, and supplies the same to theAD/DA converter 128. Additionally, the encoding may include a MIMOtransmission process and an OFDM demodulation process, for example.

The control unit 142 (mode control unit) controls transmissionprocessing and reception processing of the base station 10, shifting ofthe operation mode of the relay device 30, or the like. For example,when information indicating the relay device 30 to be used for thecommunication with the base station 10 is received from the mobileterminal 20, if the relay device 30 is operating in the sleep mode, thecontrol unit 142 issues an instruction by the L1/L2 signaling to shiftto the active mode.

Here, since the relay device 30 receives the L1/L2 signaling accordingto the DRX cycle even when operating in the sleep mode, it shifts to theactive mode according to the instruction from the base station 10included in the L1/L2 signaling. As a result, the base station 10 andthe mobile terminal 20 can communicate via the relay device 30 which hasshifted to the active mode.

2-4. Operation of Communication System

In the foregoing, the configurations of the relay device 30, the mobileterminal 20, and the base station 10 have been described. Next, theoperation of the communication system 1 according to the firstembodiment will be described with reference to FIG. 10.

FIG. 10 is a sequence chart showing the operation of the communicationsystem 1 according to the first embodiment. As shown in FIG. 10, eachrelay device 30 transmits the reference signal (S404). Here, the relaydevice 30C operating in the sleep mode transmits the reference signalaccording to the DTX cycle, and thus the frequency of transmitting thereference signals is lower than the relay device 30A operating in theactive mode.

Then, the relay device determination unit 236 of the mobile terminal 20acquires the reception intensity of the reference signal transmittedfrom each relay device 30 (magnitude of correlation), and determines arelay device 30 to be used for the communication with the base station10 (S408). Additionally, the present sequence chart shows an examplewhere the relay device 30C is determined as the relay device 30 to beused for the communication with the base station 10.

Then, the mobile terminal 20 notifies the base station 10 of informationindicating the determined relay device 30C using the control channel ofthe uplink of the direct link (S412). Since the relay mode 30C notifiedby the mobile terminal 20 is in the sleep mode, the control unit 142 ofthe base station 10 transmits a control signal indicating shift from thesleep mode to the active mode using the L1/L2 signaling (S416).

The relay device 30C operating in the sleep mode monitors the L1/L2signaling according to the DRX cycle, and in the case an instruction toshift from the sleep mode to the active mode is issued in the L1/L2signaling, shifts to the active mode (S420). Then, the relay device 30Cstarts transmitting the reference signal at normal intervals (S424).

Subsequently, when a reference signal is received from the relay device30C which has shifted to the active mode, the synchronization unit 232of the mobile terminal 20 establishes synchronization of the radio frame(S428). Then, the mobile terminal 20 and the base station 10 are enabledto perform uplink communication and downlink communication using therelay device 30C (S432). Additionally, when the mobile terminal 20 thatis synchronised with the relay device 30C no longer exists, the relaydevice 30C may shift from the active mode to the sleep mode based on aninstruction from the base station 10.

3. Second Embodiment

In the foregoing, the first embodiment of the present invention has beendescribed. Next, the second embodiment of the present invention will bedescribed. Unlike the relay device 30 of the first embodiment, a relaydevice 30′ of the second embodiment does not transmit a reference signalin the sleep mode. Therefore, the second embodiment is different fromthe first embodiment in other configurations as well, as will bedescribed below.

3-1. Configuration of Relay Device

First, the configuration of the relay device 30′ according to the secondembodiment will be described with reference to FIG. 11.

FIG. 11 is a functional block diagram showing the configuration of therelay device 30′ according to the second embodiment. As shown in FIG.11, the relay device 30′ includes a plurality of antennas 320 a to 320n, an analogue processing unit 324, an AD/DA converter 328, and adigital processing unit 330, and performs L2 relay. Also, the digitalprocessing unit 330 includes a synchronization unit 332, a decoder 334,a buffer 338, an encoder 340, a control unit 342, and a DRX cycleholding unit 344.

Unlike the relay device 30 according to the first embodiment, the relaydevice 30′ according to the second embodiment does not possess a DTXcycle holding unit 346. That is, in the sleep mode, the relay device 30′performs intermittent reception according to the DRX cycle, but does nottransmit a reference signal.

Additionally, the plurality of antennas 320 a to 320 n, the analogueprocessing unit 324, the AD/DA converter 328, and the like can beconfigured to be substantially the same as the relay device 30 accordingto the first embodiment, and detailed description thereof is omitted.

3-2. Configuration of Mobile Terminal

Next, the configuration of a mobile terminal 20′ according to the secondembodiment will be described with reference to FIG. 12.

FIG. 12 is a functional block diagram showing the configuration of themobile terminal 20′ according to the second embodiment. As shown in FIG.12, the mobile terminal 20′ includes a plurality of antennas 220 a to220 n, an analogue processing unit 224, an AD/DA converter 228, and adigital processing unit 230. Also, the digital processing unit 230includes a synchronization unit 232, a decoder 234, a relay devicedetermination unit 236, a transmission data generation unit 238, anencoder 240, a control unit 242, and a DRX cycle holding unit 244.

As described above, the relay device 30′ according to the secondembodiment does not transmit a reference signal in the sleep mode.Therefore, the mobile terminal 20′ according to the second embodimentdoes not possess a DTX cycle holding unit 346. That is, in the sleepmode, the mobile terminal 20′ according to the second embodimentperforms intermittent reception according to the DRX cycle but does notperform intermittent reception according to the DTX cycle.

Additionally, the plurality of antennas 220 a to 220 n, the analogueprocessing unit 224, the AD/DA converter 228, and the like can beconfigured to be substantially the same as the mobile terminal 20according to the first embodiment, and detailed description thereof isomitted.

3-3. Configuration of Base Station

Next, the configuration of a base station 10′ according to the secondembodiment will be described with reference to FIG. 13.

FIG. 13 is a functional block diagram showing the configuration of thebase station 10′ according to the second embodiment. As shown in FIG.13, the base station 10′ includes groups of antennas 122 a to 122 n forrespective sectors, an analogue processing unit 124, an AD/DA converter128, and a digital processing unit 130.

A cell formed by the base station 10′ is formed from a plurality ofsectors, and each of the groups of antennas 122 a to 122 n performstransmission to the corresponding sector and reception from thecorresponding sector. Additionally, the analogue processing unit 124,the AD/DA converter 128, and the like have to be compatible with sectorcommunication, but basically they can be configured to be substantiallythe same as the base station 10 according to the first embodiment, anddetailed description thereof is omitted.

The digital processing unit 130 includes a decoder 134, an encoder 140,a control unit 142, a terminal location acquisition unit 152, a relaydevice location holding unit 154, and a selection unit 156.

The terminal location acquisition unit 152 acquires informationindicating the existing location of the mobile terminal 20′.Specifically, the terminal location acquisition unit 152 detects thearrival direction of a radio signal, that is the direction in which themobile terminal 20′ exists, based on which of the groups of antennas 122a to 122 n received the radio signal transmitted from the mobileterminal 20′. Additionally, the detection method of the existingdirection is not limited to the above, and an arrival directionestimation algorithm capable of electronically obtaining the directivitymay also be used.

Also, the terminal location acquisition unit 152 detects the distancebetween the mobile terminal 20′ and the base station 10′ based on thereception intensity of a radio signal transmitted from the mobileterminal 20′. For example, the terminal location acquisition unit 152may compare the transmission power of a radio signal by the mobileterminal 20′ and the reception intensity of the radio signal at the basestation 10′ and detect the distance between the mobile terminal 20′ andthe base station 10′ from the propagation loss. Additionally, a casewhere the mobile terminal 20′ individually sets the transmission poweris also conceivable, and thus a mobile terminal 20′ may transmitinformation indicating the transmission power and the terminal locationacquisition unit 152 may obtain the transmission power of a radio signalfrom this information. Additionally, the relationship between thepropagation loss in the free space and the distance is expressed as thefollowing formula 1, for example.L=20 log 10(4πd/λ)[dB]  (Formula 1)

d: distance

λ: wavelength

The terminal location acquisition unit 152 can perceive the existinglocation of the mobile terminal 20′ by detecting the existing directionand the distance to the base station 10′ of the mobile terminal 20′ inthe above manner. Additionally, since the reception timing by the basestation 10′ of a signal transmitted by the mobile terminal 20′ differsaccording to the distance between the base station 10′ and the mobileterminal 20′, the terminal location acquisition unit 152 may estimatethe distance to the mobile terminal 20′ based on this reception timing.

Furthermore, the relay device location holding unit 154 holds locationinformation of each relay device 30′. The base station 10′ may acquirethe location information of each relay device 30′ by detecting thedistance between the existing direction and the distance to the basestation 10′ of a relay device 30′ in the above manner based on a signaltransmitted from the relay device 30′ in the active mode, for example.

The selection unit 156 refers to the location information of the mobileterminal 20′ acquired by the terminal location acquisition unit 152 andthe location information of each relay device 30′ held by the relaydevice location holding unit 154, and selects a relay device 30′suitable for communication with the mobile terminal 20′. For example,the selection unit 156 selects a relay device 30′ that is nearest to themobile terminal 20′.

The control unit 142 issues an instruction to shift to the active modeby L1/L2 signaling, in the case the relay device 30′ selected by theselection unit 156 is operating in the sleep mode. The relay device 30′receives the L1/L2 signaling according to the DRX cycle even whenoperating in the sleep mode, and thus shifts to the active modeaccording to the instruction from the base station 10′ included in theL1/L2 signaling. As a result, the relay device 30′ starts transmissionof reference signal, and the mobile terminal 20′ is allowed to determinethe relay device 30′ to be used for the communication with the basestation 10′ based on the reference signal transmitted by the relaydevice 30′.

3-4. Operation of Communication System

In the foregoing, the configurations of the relay device 30′, the mobileterminal 20′, and the base station 10′ have been described. Next, theoperation of the communication system according to the second embodimentwill be described with reference to FIG. 14.

FIG. 14 is a sequence chart showing the operation of the communicationsystem according to the second embodiment. As shown in FIG. 14, therelay device 30′A operating in the active mode transmits a referencesignal and the relay device 30′C operating in the sleep mode does nottransmit a reference signal (S504). When the mobile terminal 20′transmits a radio signal here (S508), the terminal location acquisitionunit 152 of the base station 10′ estimates the location of the mobileterminal 20′ by detecting the arrival direction of this radio signal andthe distance to the mobile terminal 20′ (S512).

Furthermore, the selection unit 156 of the base station 10′ selects therelay device 30′ that is near the mobile terminal 20′ (S516). Here, itis assumed that the selection unit 156 of the base station 10′ selectedthe relay device 30′C. In this case, since the relay device 30′C is inthe sleep mode, the control unit 142 of the base station 10′ transmits acontrol signal indicating shift from the sleep mode to the active modeusing L1/L2 signaling (S520).

The relay device 30′C operating in the sleep mode monitors the L1/L2signaling according to the DRX cycle, and in the case an instruction toshift from the sleep mode to the active mode is issued in the L1/L2signaling, shifts to the active mode (S524). Then, the relay device 30′Cstarts transmission of the reference signal (S528).

Then, the relay device determination unit 236 of the mobile terminal 20′acquires the reception intensity of the reference signal transmittedfrom each relay device 30′ (magnitude of correlation), and determines arelay device 30′ to be used for the communication with the base station10′ (S532). Here, it is assumed that the relay device selection unit 236of the mobile terminal 20′ determined to use the relay device 30′C forthe communication with the base station 10′. In this case, the mobileterminal 20′ notifies the base station 10′ of the information indicatingthe determined relay device 30′C using the control channel of the uplinkof the direct link (S536).

As a result, the mobile terminal 20′ and the base station 10′ areenabled to perform uplink communication and downlink communication usingthe relay device 30′C based on the scheduling by the base station 10′(S540). Additionally, when the mobile terminal 20′ that is synchronisedwith the relay device 30′C no longer exists, the relay device 30′C mayshift from the active mode to the sleep mode based on an instructionfrom the base station 10′.

3-5. Modified Example 1

In the foregoing, the second embodiment has been described. However, theacquisition method of the location information of the mobile terminal20′ by the terminal location acquisition unit 152 of the base station10′ is not limited to the above example. In the following, modifiedexamples 1 to 4 of the acquisition method of the location information ofthe mobile terminal 20′ by the terminal location acquisition unit 152will be described.

FIG. 15 is an explanatory diagram showing a modified example 1 of theacquisition method of the location information of the mobile terminal20′. As shown in FIG. 15, according to the modified example 1, althoughthe mobile terminal 20′ belongs to the base station 10′A, it alsoreceives the reference signals transmitted by the base stations 10′B and10′C. Then, the mobile terminal 20′ measures the reception intensity ofthe reference signal transmitted by each base station 10′ and transmitsreception intensity information obtained by the measurement to the basestation 10′A.

The terminal location acquisition unit 152 of the base station 10′Aestimates the location of the mobile terminal 20′ based on the receptionintensity information received from the mobile terminal 20′. Forexample, the terminal location acquisition unit 152 may estimate thedistance from each base station 10′ to the mobile terminal 20′ based onthe reception intensity at the mobile terminal 20′ of the referencesignal transmitted from each base station 10′, and estimate the locationsatisfying each of the estimated distances as the location of the mobileterminal 20′.

3-6. Modified Example 2

FIG. 16 is an explanatory diagram showing a modified example 2 of theacquisition method of the location information of the mobile terminal20′. As shown in FIG. 16, according to the modified example 2, not onlythe base station 10′A but also the base stations 10′B and 10′C receivethe radio signal transmitted by the mobile terminal 20′ belonging to thebase station 10′A. Then, each base station 10′ estimates the directionand distance of the mobile terminal 20′ based on the radio signalreceived from the mobile terminal 20′. Furthermore, the base stations10′B and 10′C transmit information related to the estimated location ofthe mobile terminal 20′ to the base station 10′A via the backbonenetwork 12.

Accordingly, the terminal location acquisition unit 152 of the basestation 10′A can acquire the location information of the mobile terminal20′ by combining the pieces of information related to the location ofthe mobile terminal 20′ estimated by the base stations 10′.Additionally, the base stations 10′B and 10′C may transmit to the basestation 10′A only the reception intensity of the radio signal receivedfrom the mobile terminal 20′, and the base station 10′A may estimate thelocation of the mobile terminal 20′ from this reception intensityinformation.

3-7. Modified Example 3

FIG. 17 is an explanatory diagram showing a modified example 3 of theacquisition method of the location information of the mobile terminal20′. As shown in FIG. 17, according to the modified example 3, themobile terminal 20′ receives the reference signals transmitted from thebase station 10′A and each relay device 30′ operating in the activemode. Then, the mobile terminal 20′ measures the reception intensity ofthe reference signals transmitted by the base station 10′A and eachrelay device 30′ and transmits reception intensity information obtainedby the measurement to the base station 10′A.

The terminal location acquisition unit 152 of the base station 10′Aestimates the location of the mobile terminal 20′ based on the receptionintensity information received from the mobile terminal 20′. Forexample, the terminal location acquisition unit 152 may estimate thedistances from the base station 10′A and each relay device 30′ to themobile terminal 20′ based on the reception intensity at the mobileterminal 20′ of the reference signals transmitted from the base station10′A and each relay device 30′, and estimate the location satisfyingeach of the estimated distances as the location of the mobile terminal20′.

3-8. Modified Example 4

Furthermore, according to a modified example 4, the mobile terminal 20′possesses a GPS (Global Positioning System) receiving function. Themobile terminal 20′ acquires the location information of the mobileterminal 20′ using this GPS receiving function, and transmits theacquired location information to the base station 10′. As a result, theterminal location acquisition unit 152 of the base station 10′ isenabled to obtain the location information of the mobile terminal 20′.

4. Summary

As described above, according to the first embodiment of the presentinvention, a relay device 30 operating in the sleep mode also transmitsreference signals according to the DRX cycle. Therefore, the mobileterminal 20 can receive the reference signal not only from a relaydevice 30 operating in the active mode but also from a relay device 30operating in the sleep mode. Accordingly, the mobile terminal 20 isenabled to determine a relay device 30 to be used for the communicationwith the base station 10 from a plurality of relay devices 30 includingthe relay device 30 operating in the sleep mode, and to performcommunication via the determined relay device 30.

Also, according to the second embodiment of the present invention, thebase station 10′ causes a relay device 30′ operating in the sleep modenear the mobile terminal 20′ to shift to the active mode. Accordingly,the mobile terminal 20′ becomes enabled to receive the reference signalalso from the relay device 30′ which was operating in the sleep mode,and to more appropriately determine the relay device 30′ to be used forthe communication with the base station 10′.

The preferred embodiments of the present invention have been describedabove with reference to the accompanying drawings, whilst the presentinvention is not limited to the above examples, of course. A personskilled in the art may find various alternations and modificationswithin the scope of the appended claims, and it should be understoodthat they will naturally come under the technical scope of the presentinvention.

For example, the steps of the processing of the communication system 1of the present specification do not necessarily have to be processedchronologically according to the order described as the sequence chart.For example, the steps of the processing of the communication system 1may be processed according to an order different from the order shown asthe sequence chart or may be processed in parallel.

Furthermore, a computer program for causing hardware, such as a CPU, aROM, a RAM, or the like, installed in the base station 10, the mobileterminal 20, or the relay device 30 to realize the same function as eachcomponent of the above-described base station 10, the mobile terminal20, or the relay device 30 can also be created. Furthermore, a storagemedium storing the computer program is also provided.

The invention claimed is:
 1. A communication system comprising: a mobileterminal; one or more communication devices; and a base station forcommunicating with at least one of the mobile terminal and one or morecommunication devices, wherein the mobile terminal includes a receiverconfigured to receive at least one of a first reference signal and asecond reference signal transmitted from the one or more communicationdevices; and circuitry configured to identify a communication devicebased on the at least one of the first reference signal and the secondreference signal received by the receiver from the one or morecommunication devices, each of the one or more communication devicestransmits the first reference signal according to a first cycle whenoperating in a power save mode and transmits the second reference signalaccording to a second cycle when operating in an active mode, the firstcycle being longer than the second cycle, and the base station includescircuitry configured to select a communication device among the one ormore communication devices, and issue an instruction to shift to theactive mode in a case the selected communication device is operating inthe power save mode.
 2. The communication system according to claim 1,wherein the circuitry of the base station is configured to: acquirelocation information corresponding to the mobile terminal; and select acommunication device among the one or more communication devices basedon the location information corresponding to the mobile terminal.
 3. Thecommunication system according to claim 2, wherein the circuitry of thebase station is configured to acquire the location informationcorresponding to the mobile terminal based on an arrival direction and areception intensity of a signal transmitted from the mobile terminal. 4.The communication system according to claim 2, wherein the mobileterminal transmits to the base station reception intensity informationof each signal transmitted from a plurality of base stations, and thecircuitry of the base station is configured to acquire the locationinformation of the mobile terminal based on the reception intensityinformation of each of the plurality of base stations received from themobile terminal.
 5. The communication system according to claim 2,wherein a plurality of base stations transmit to the base stationreception intensity information of a signal transmitted by the mobileterminal, and the circuitry of the base station is configured to acquirethe location information corresponding to the mobile terminal based onreception intensities of the signal transmitted by the mobile terminalat the plurality of base stations.
 6. The communication system accordingto claim 2, wherein the mobile terminal transmits to the base stationreception intensity information of each signal transmitted from the basestation and the one or more communication devices, and the circuitry ofthe base station is configured to acquire the location information ofthe mobile terminal based on the reception intensity informationreceived from the mobile terminal.
 7. The communication system accordingto claim 2, wherein the mobile terminal transmits to the base stationthe location information of the mobile terminal estimated by GPS, andthe circuitry of the base station is configured to acquire the locationinformation of the mobile terminal transmitted from the mobile terminal.8. A communication control method comprising: receiving, by a mobileterminal, at least one of a first reference signal and a secondreference signal transmitted from one or more communication devices;determining a communication device based on the at least one of thefirst reference signal and the second reference signal received from theone or more communication devices, wherein each of the one or morecommunication devices transmits the first reference signal according toa first cycle when operating in a power save mode and transmits thesecond reference signal according to a second cycle when operating in anactive mode, the first cycle being longer than the second cycle; andissuing an instruction to the determined communication device to shiftto the active mode in a case the determined communication device isoperating in the power save mode.
 9. A mobile terminal comprising: areceiver configured to receive at least one of a first reference signaland a second reference signal transmitted from one or more communicationdevices, wherein each of the one or more communication devices transmitsthe first reference signal according to a first cycle when operating ina power save mode and transmits the second reference signal according toa second cycle when operating in an active mode, the first cycle beinglonger than the second cycle.
 10. The mobile terminal of claim 9,further comprising: a transmitter configured to transmit, to a basestation, reception intensity information corresponding to each signaltransmitted from a plurality of base stations.
 11. The mobile terminalof claim 9, further comprising: a transmitter configured to transmit, toa base station, reception intensity information corresponding to eachsignal transmitted from the base station and the one or morecommunication devices.
 12. The mobile device of claim 9, furthercomprising: a transmitter configured to transmit, to a base station,location information corresponding to the mobile device estimated byGPS.
 13. The communication device of claim 10, wherein the communicationinterface is configured to: perform discontinuous reception in a casethat the communication device is operating in the power save mode; andreceive an indication from the base station while performing thediscontinuous reception.
 14. The mobile terminal of claim 9, wherein thefirst cycle is a discontinuous transmission (DTX) cycle.
 15. The mobileterminal of claim 9, wherein the first cycle is longer than a length ofa sub-frame.
 16. The mobile terminal of claim 9, wherein the first cycleis longer than a length of 10 sub-frames.
 17. A communication device,comprising: a communication interface configured to communicate with atleast one of a base station and a mobile terminal; and circuitryconfigured to set an operation mode of the communication device to apower save mode in which the communication device transmit a firstreference signal according to a first cycle or an active mode in whichthe communication device transmits a second reference signal accordingto a second cycle, the first cycle being longer than the second cycle.18. The communication device of claim 17, wherein the first cycle is adiscontinuous transmission (DTX) cycle.
 19. The communication device ofclaim 17, wherein the first cycle is longer than a length of asub-frame.
 20. The communication device of claim 17, wherein the firstcycle is longer than a length of 10 sub-frames.
 21. The communicationdevice of claim 17, wherein the first reference signal is used forsynchronization.
 22. A base station configured to communicate with oneor more communication devices, the base station comprising: circuitryconfigured to acquire location information corresponding to a mobileterminal; select a communication device among the one or morecommunication devices base on the acquired location information; andissue an instruction to shift the selected communication device toactive mode, in a case the selected communication device is operating ina power save mode, wherein the selected communication device operates inone of the power save mode in which the communication device transmits afirst reference signal according to a first cycle or the active mode inwhich the communication device transmits a second reference signalaccording to a second cycle, the first cycle being longer than thesecond cycle.
 23. The base station according to claim 22, wherein thecircuitry is configured to acquire the location informationcorresponding to the mobile terminal based on an arrival direction of asignal and a reception intensity of the signal transmitted from themobile terminal.
 24. The base station according to claim 22, wherein thecircuitry is configured to: receive, from the mobile terminal, receptionintensity information corresponding to each signal transmitted from aplurality of base stations; and acquire the location informationcorresponding to the mobile terminal based on the reception intensityinformation of each of the plurality of base stations.
 25. The basestation according to claim 22, wherein the circuitry is configured to:receive, from a plurality of base stations, reception intensityinformation of a signal transmitted by the mobile terminal; and acquirethe location information corresponding to the mobile terminal based onreception intensities of the signal.
 26. The base station according toclaim 22, wherein the circuitry is configured to: receive, from themobile station, reception intensity information of each signaltransmitted from the one or more communication devices; and acquire thelocation information corresponding to the mobile terminal based on thereception intensity information.
 27. The base station according to claim22, wherein the circuitry is configured to: receive, from the mobilestation, the location information corresponding to the mobile terminalestimated by GPS; and acquire the location information corresponding tothe mobile terminal based on the received location information.
 28. Themobile terminal of claim 22, wherein the first cycle is a discontinuoustransmission (DTX) cycle.
 29. The mobile terminal of claim 22, whereinthe first cycle is longer than a length of a sub-frame.
 30. The mobileterminal of claim 22, wherein the first cycle is longer than a length of10 sub-frames.
 31. The mobile terminal of claim 22, wherein the firstreference signal is used for synchronization.